# std::is_polymorphic

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Defined in header `<type_traits>` |  |
template< class T > struct is_polymorphic; |  | (since C++11)

`std::is_polymorphic` is a UnaryTypeTrait.

If `T` is a polymorphic class (that is, a non-union class that declares or inherits at least one virtual function), provides the member constant `value` equal to true. For any other type, `value` is false.

If `T` is an incomplete non-union class type, the behavior is undefined.

If the program adds specializations for `std::is_polymorphic` or `std::is_polymorphic_v`, the behavior is undefined.

### Template parameters

T | - | a type to check

### Helper variable template

template< class T > constexpr bool is_polymorphic_v = is_polymorphic<T>::value; |  | (since C++17)

## Inherited from std::integral_constant

### Member constants

value [static] | true if `T` is a polymorphic class type, false otherwise (public static member constant)

### Member functions

operator bool | converts the object to bool, returns value (public member function)
operator() (C++14) | returns value (public member function)

### Member types

Type | Definition
`value_type` | bool
`type` | std::integral_constant<bool, value>

### Possible implementation

    namespace detail
    {
        template<class T>
        std::true_type detect_is_polymorphic(
            decltype(dynamic_cast<const volatile void*>(static_cast<T*>(nullptr)))
        );
        template<class T>
        std::false_type detect_is_polymorphic(...);
    } // namespace detail

    template<class T>
    struct is_polymorphic : decltype(detail::detect_is_polymorphic<T>(nullptr)) {};

### Example

Run this code

    #include <type_traits>

    struct A { int m; };
    static_assert(!std::is_polymorphic_v<A>);

    struct B { virtual void foo(); };
    static_assert(std::is_polymorphic_v<B>);

    struct C : B {};
    static_assert(std::is_polymorphic_v<C>);

    struct D { virtual ~D() = default; };
    static_assert(std::is_polymorphic_v<D>);

    // Uses inheritance, but not the virtual keyword:
    struct E : A {};
    static_assert(!std::is_polymorphic_v<E>);

    struct F : virtual A {};
    static_assert(!std::is_polymorphic_v<F>);

    struct AX : A {};
    struct AY : A {};
    struct XY : virtual AX, virtual AY {};
    static_assert(!std::is_polymorphic_v<XY>);

    int main() {}

### Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR | Applied to | Behavior as published | Correct behavior
--- | --- | --- | ---
LWG 2015 | C++11 | the behavior was undefined if `T` is an incomplete union type | the base characteristic is std::false_type in this case

### See also

is_class (C++11) | checks if a type is a non-union class type (class template)
is_abstract (C++11) | checks if a type is an abstract class type (class template)
has_virtual_destructor (C++11) | checks if a type has a virtual destructor (class template)

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